The present invention relates to leveling instruments of the type used for determining inclination with respect to a gravity field and, more particularly, to such an instrument which responds electrically to inclinations and relies for its operation on detection of the position of a bubble within a vial.
The desirability of a leveling device which produces an electrical output indicative of its inclination with respect to a gravity field, has been recognized for some time. The electrical output of such an electronic level can be used, for example, to energize lamps, audio devices, etc., as a signal of the achievement of a desired inclination. Such an electrical output can also be used in a feedback arrangement to cause the inclination of a structure to be changed automatically to coincide with a desired inclination.
It has been proposed in the past that such an instrument which responds electrically include a pendulum for sensing inclination with respect to a gravity field. U.S. Pat. Nos. 723,526; 3,233,235 and 3,861,052 describe exemplary instruments of this type. Such instruments, however, are generally complex and fairly delicate. Moreover, they typically are not very sensitive or accurate in view of the resistance to pendulum movement provided by the pendulum mounting, and the fact that as a practical matter a substantial "swing" of the pendulum is required before its movement can be detected.
In view of the problems inherent in pendulum type electronic levels, most of those working in the field have turned to the common bubble-within-a-vial arrangement in an effort to provide satisfactory sensing of inclination. A vial indicator has the advantage of being quite simple, and the position of the bubble can be detected relatively easily with optical radiation. In this connection, the term "optical radiation" as used herein is meant to encompass not only electromagnetic radiation in the visible spectrum, but also that radiation adjacent thereto in the spectrum, such as infrared and near ultra-violet radiation, having basically the same transmission properties as visible radiation. Vial indicators have the additional advantage of providing a faster response to changes of attitude than most pendulum arrangements Examples of electronic levels relying on the detection of the location of a bubble within a vial by optical radiation can be found in U.S. Pat. Nos. 2,252,727; 3,324,564; and 3,371,424.
There are problems associated with bubble vial electronic levels, however, which have prevented the same from being accepted to any great degree. One problem has to do with their sensitivity. To obtain adequate optical radiation variations, most of such devices rely on the reflection, refraction, or absorption of radiation by the meniscus of the bubble. Such arrangements are extremely sensitive to even the slightest mechanical shock or vibration since any surface change at the bubble meniscus will affect the light. Thus, false alarms or readings can be common. Moreover, tipping of a bubble vial electronic level in a direction different than that whose inclination is to be measured can result in false readings. That is, the bubble configuration and movement is sensitive to the orientation of the instrument in all directions with respect to the gravity field, even though typically it is inclination only in one direction which is to be monitored or measured. Again, erroneous readings can result.